For oat flour, in a food processor bowl or blender container place 1/2 cup of the oats. Cover and process or blend until reduced to a powder. Transfer to a large bowl. Repeat with the remaining oats, 1/2 cup at a time. (You should have about 1-1/2 cups of oat flour.)
Add all-purpose flour, brown sugar, and baking powder to the oat flour. Stir until thoroughly combined. Using a pastry blender, cut in the butter until the mixture resembles coarse crumbs.
Make a well in the center of the flour mixture. Add cold water all at once. Using a fork, stir until the mixture can be gathered into a ball. Divide dough in half.
On a lightly floured surface roll each half of dough to 1/8-inch thickness. Using a 2-inch round cookie cutter, cut dough into rounds. Place on an ungreased baking sheet. Using a fork, prick crackers all over.
Bake in a 350 degree F oven for 15 to 18 minutes or until firm. Cool crackers on a wire rack. Makes 84 crackers.